It has been known for many years in the field of fibre-reinforced composite materials to provide a prepreg which comprises a layer of fibrous reinforcement impregnated with a structural polymer resin. The amount of structural polymer resin is carefully matched with the amount of fibrous reinforcement. Accordingly, the prepreg may be used in a method for forming a fibre-reinforced composite material, in which a multilayer stack of prepregs is provided having a desired shape and configuration, and then is subjected to heating so that the structural polymer resin melts and then solidifies to form a single unified resin matrix in which the fibrous reinforcement is disposed in the desired fibre orientation. The amount of resin in the stack is sufficient to make a fibre-reinforced structural article from the stack of prepregs which has the desired mechanical properties. Typically, the structural polymer resin is a thermosetting resin, most typically an epoxy resin, which is cured to form the solid resin matrix. The fibres may be selected from a variety of materials, most typically comprising glass fibres or carbon fibres.
It is very well known to provide prepregs in which the structural polymer resin is fully impregnated into the layer of fibrous reinforcement. This provides the outer major surfaces of the prepreg with a resin surface, distributes the fibres substantially uniformly throughout the prepreg resin so that the fibres are uniformly embedded within the resin and minimise the presence of inadvertent voids within the initial resin layer. This provides the advantage that the resin surface can be slightly tacky to assist lay up of the prepregs into the mould by supporting the prepreg at a desired position as a result of the adhesion of the prepreg by the tacky resin surface to an adjacent surface. In addition, the full impregnation of the fibrous reinforcement obviates the need for the structural polymer resin to flow significantly the curing phase, and ensures that the fibres wet out uniformly during the curing phase.
However, one particular problem with fully impregnated prepregs is that when a stack of such prepregs is formed, air can be trapped between the adjacent prepreg plies, with the result that in the final cured resin matrix of the fibre reinforced composite material inter-ply voids can exist. The presence of these voids can significantly reduce the mechanical properties of the composite material. As the layers of fully impregnated prepregs are progressively built up to form a multilayer stack thereof during the prepreg lay-up process, air can be trapped between the adjacent prepreg layers. The tackiness of the resin surfaces of the adjacent prepreg layers increases the possibility of air being trapped between the plies at the prepreg interfaces.
In addition, in order to provide such a fully impregnated prepreg with low tack, the matrix resin viscosity affects the drapeability of the prepreg, namely the ability of the prepreg to adopt a three dimensional shape of a mould surface. As a rule a higher drape resin has higher surface tack. There is a need in the art for a prepreg which has a combination of low tack and high drape.
It is known to use prepregs for manufacturing a wide variety of products, having a wide variety of thicknesses, shapes and volumes, and desired mechanical properties. One particular application for composite materials is the manufacture of structural elements in the form of elongate spars or beams which are required to exhibit a high mechanical stiffness and compressive strength. A “sparcap” is an elongate spar laminate which is incorporated into a particular layup for manufacturing a wind turbine blade, as is known in the art. The sparcap is an outer elongate capping laminate layer on opposite sides of a central structural element to form an elongate beam of enhanced mechanical strength similar to an “I” beam construction. For such spars or beams, in order to maximise the mechanical stiffness and compressive strength, it is desired to provide fibres which primarily are oriented along the direction of the elongate spar or beam, in particular are unidirectional fibres.
In contrast, to provide composite materials having a sheet-like construction, or providing a torsional strength, it would be desirable to provide biaxially oriented fibres.
The typical elongate members forming the main structural reinforcement in wind energy and tidal spar sections are 15-70 m long and 10 to 100 mm in thickness. They are made predominately from uni-directional fibres to give the longitudinal strength and stiffness. A typical spar cap section may contain 85 to 100% unidirectional fibre with some off axis material for shear transfer, torsional stiffness, and spar cap buckling stability depending on the design. Typically the thickness and or the width may change along the length of the spar to match the required design stiffness and stresses. There may also be a change in thickness in any given cross section to provide a suitable geometric and load transition to the shear webs. In addition to the changes in thickness the geometry of the spar cap can have curvature in one or more directions to match the desired aerodynamic profile and make beneficial use of curvature to reduce to reduce the tendency for the spar cap to buckle under load.
With the changing geometry and thickness the laminate is typically built up from plies of fibre reinforcement to give an economic balance of mould deposition rate, material cost, and geometrical flexibility in the design process. Typically glass reinforcement fabrics or prepregs with 500-2400 gsm areal fibre weight, or carbon reinforcement fabric or prepregs 100-900 gsm areal weight are used. These plies, or layers, can be pre-cut to geometrical shapes or used in roll format. With the changing curvature some shear is usually needed to accommodate the changing lengths between adjacent fibres in a sheet or roll of fibre reinforced material. The width and thickness of a given fibre reinforcement is typically limited to drape the material into the mould shape without significant fibre buckling or inducing fibre waviness
In comparison to stitched, multiaxial and woven materials, multiple layers of unidirectional fibres will nest and pack efficiently leaving limited free volume space, which reduces air and resin permeability. Carbon fibres typically are smaller in diameter (7 micron) than glass fibres (17-24 micron) and therefore such carbon fibre stacks are harder to process reliably as they are inherently less permeable to air and resin.
In the VARTM (Vacuum Assisted Resin Transfer Moulding) method, dry fibres are laid into a mould and then impregnated with a low cost low viscosity liquid resin. The fibres are usually pre-stitched and or stabilised with thermoplastic binder to allow the fibre to be handled into the mould and to provide a controlled spacing between the fibres to form gaps to assist the flow of the resin. This adds both areas of unreinforced resin material and parasitic weight into the spar. The fibre support structure causes waviness of the fibres, reducing the final load carrying capacity, especially in compression. This method has been successfully used to produce current generations of glass fibre wind turbines where the design usually is stiffness limited and the laminate does not need the highest compression strength. Higher material performance is required for larger blades and newer generations of more aerodynamic efficient turbines where the spar cap spacing has been reduced to give a thinner blade section with improved aerodynamic efficiency. Here higher performance glass fibre or carbon fibre laminates are required.
Thick sections of carbon fibre have proved difficult to infuse reliably with the VARTM method due to the low permeability of the reinforcement. In the carbon spar design the compression strength becomes a key design driver. Economic savings are possible by using the improved material properties of prepreg materials which allow pure collimated fibre to be used.
The higher viscosity semi-solid prepreg resin then retains the fibres straighter in a collimated format during the prepreg manufacture and layup process to give laminates with less inherent fibre waviness and higher compressive strength. In addition, it is possible to select higher molecular weight materials and toughening additives to formulate prepreg resins to have both a high resin modulus and a good balance of toughness and strength to further enhance compressive properties and fatigue resistance.
To further maximise the strength it is desirable to remove any intralaminar and interlaminar void defects in thick, elongate spar cap sections. Traditionally this was done in the aerospace industry using high pressure autoclaves to compress any entrapped air and volatiles from the material. A number of out-of autoclave prepreg technologies have subsequently been developed to use vacuum only processing, in applications where the cost of using an autoclave becomes prohibitive for producing large sections such as wind turbines and tidal spars.
These current out-of autoclave materials which use partially impregnated or dry fibre layers provide good results in laminates containing a mixture of fibre orientations because the fibres bridge over themselves to form natural air conduits. These materials do not provide laminates with very low void levels, high levels of fibre alignment and tolerance to both high and low workshop temperature conditions when building large, thick laminates out of predominately uni-directional materials.
To improve the economics of producing these higher loaded turbine sections it is desirable to provide a drapeable, prepreg material suitable for direct layup, or off line preforming, in a wider range of ambient temperature conditions which can be cured using out-of-autoclave vacuum processing methods to give well aligned fibre reinforced laminates with low intralaminar and interlaminar void defects
For an impregnated prepreg to have high drape the resin should be of a low viscosity with a low storage modulus to allow the draping shear to be accommodated as viscous resin flow without the prepreg tending to spring back to its original position.
It is known to provide a prepreg with a resin that has a low tack and a high viscosity and storage modulus. When a laminated stack of such prepregs is formed, and subjected to a vacuum de-bulking step to remove air, the resin maintains a degree of surface texture to provide air venting paths to eliminate trapped air during the de-bulking cycle. During moulding the resin then flows at and gives a homogenous material.
Such a material with a high viscosity resin is well suited to producing thick laminates when no significant curvature is required. To drape the product a degree of heat is needed to soften the resin. Heating lowers the viscosity of the resin but then the resin becomes prone to flowing and the air channels can be lost. This flow can occur before the vacuum is applied, in particular with some pre-flow of the resin during the lamination phase often resulting from variations in pressure and contact points which cause locked-in air regions. On applying the vacuum the softer resin can flow, which closes the remaining channels thereby reducing permeability of the laminate before the air has had time to be drawn out of the laminate stack.
The lower viscosity prepreg can then become very sensitive to its handling and any de-bulks before commencing cure. If a vacuum cycle is initiated and abandoned the material can consolidate and any surface texture venting paths can be lost. This is often more pronounced around the perimeter edges which can see higher forces depending on the vacuum bagging detail. On removing the vacuum air can slowly leak back between the material but on reapplying the vacuum the second and subsequent times the now compacted laminate stack is much less air permeable due to the previous viscous flow of the softer prepreg resin.
As such if drape and air breathability is required the process window can be reduced leading to the need to accurately control the workshop temperature environment, adding significant cost to the manufacturing process.
In an alternative attempt to overcome this undesirable formation of inter-ply voids, it has also been to provide prepregs which are only partially impregnated with the structural polymer resin so that a layer of dry fibre reinforcement is present on one or both of the major surfaces of the prepreg. Such a known partially impregnated prepreg, or semipreg, is manufactured by the applicant and sold under the registered trade mark SPRINT®.
Furthermore, when such partially impregnated prepregs are assembled together in a multi-laminar stack to form a structural member, during vacuum consolidation of the prepregs, the multi-laminar stack of prepregs can shrink in thickness, a phenomenon known in the art as “de-lofting”. This “de-lofting” induces some out-of plane waviness to the uni-directional fibre which lowers the compressive mechanical properties, as the fibres will buckle earlier under compressive loads.
A number of prior patent specifications have addressed the problem of removing entrapped air in prepreg laminates in order to reduce the void content of the cured fibre reinforced composite material. In particular WO-A-2001/000405 discloses a partially impregnated prepreg, having dry fibres in the centre; EP-A-1128958 discloses a prepreg with a central resin layer and outer dry fibre layers; EP-A-1379376 discloses providing a non-continuous resin layer in a prepreg with a central resin layer and outer dry fibre layers; WO-A-2002/088231 discloses providing bands of resin film on the fibrous surface; EP-A-2268720 discloses providing a high viscosity rigid resin at least at the surface of the prepreg, combined with an embossed resin surface to provide an air channel; EP-A-1595689 discloses a scrim partly impressed into a resin surface of a fully impregnated prepreg; EP-A-2254936 discloses printing resin regions onto a prepreg; and WO-A-2012052272 discloses providing dry fibre channels in a prepreg surface, optionally combined with a scrim partly impressed into a resin surface of the prepreg.
All of these prior disclosures suffer from various problems in reliably and cost-effectively producing void free large dimension structural members, particularly incorporating unidirectional fibres, where the combination of high air permeability of the resin stack, high drape, low tack, low loft and high fibre collimation can be achieved to provide a structural member with high mechanical properties.
There is therefore still the need for a prepreg, which has particular application for the manufacture of structural members such as spars, in particular for wind turbine blades, which overcomes the problems of fully impregnated prepregs, scrim-coated fully impregnated prepregs and partially impregnated prepregs as discussed above.
It is also well known in the composite material art to provide a two-phase matrix resin system in which the matrix resin, as well as surrounding fibrous reinforcement, includes a second phase of fine particles which enhances the toughness, in particular the impact resistance, the fibre reinforced resin composite material. Various methods are known for incorporating the particles into the matrix resin. For example, EP-A-0274889 and EP-A-0745640 disclose incorporating particles into the matrix resin using a variety of techniques, including applying particles to the prepreg surface prior to layup and curing so that after curing the particles are concentrated in a matrix resin layer at the internal surface(s) of the cured composite corresponding to the original prepreg layers. There is no disclosure of how to provide a prepreg with a structure or properties to provide a laminated prepreg stack with increased air venting.
The present invention at least partially aims to overcome these technical problems of known prepregs for the manufacturing of structural members, in particular elongate structural members in the form of spars or beams.